In recent years there has been intense interest in the dental field in adhesives that bond to hard tissues such as dentin. Forces generated by the polymerization contraction of dental restorative materials suggest that a minimum adhesion strength for bonding restorative materials to hard surfaces in in vivo clinical procedures would be desirable. For example, M. Jensen, Polymerization Shrinkage and Microleakage International Symposium on Posterior Composite Resin Dental Restorative Materials, 234-44 (1985) reports a contraction force of 7.3 MPa. for conventional composite materials. In many instances the minimum adhesive strength has not been achieved, resulting in direct communication between dentin and the oral cavity via gaps between the dental restorative material and the cavity walls. This may be responsible, in part, for patient complaints of sensitivity and for pulpal irritation and inflammation. See, Tao, The relationship between dentin bond strengths and dentin permeability Dental Materials, Vol. 5, 133-39 (1989).
Recently a novel priming method was developed in the laboratory of the assignee of this invention and is sold commercially as ScotchBond 2.TM. Light Cure Dental Adhesive with Scotchprep.TM. Dentin Primer (commercially available from 3M). This priming method has achieved average shear strengths in vitro in excess of 20 MPa. U.S. Pat. No. 4,719,149 (Aasen et at.) describes that invention as an acid and a water-soluble film former useful for priming hard tissue (e.g., dentin). The acid has a pKa less than or equal to that of phenol. The calcium salt(s) of the acid are soluble in the film former. The film former is exemplified as comprising various difunctional and monofunctional monomers and optional cosolvents with 2-hydroxyethylmethacrylate and water being preferred.
Kusumoto et at., U.S. Pat. No. 4,535,102 discloses an adhesive coating material for a hard tissue comprising (1) a polymer having an acid value of 30 to 700 and including in recurring units a hydrophobic group and two carboxyl groups or one carboxylic anhydride group bonded to the polymer, and (2) a polymerizable vinyl compound or a mixture of said polymerizable vinyl compound and an organic titanate compound.
Engelbrecht et al., U.S. Pat. No. 4,806,381 discloses oligomeric or prepolymeric organic compounds that contain both polymerizable unsaturated groups and acid radicals, their salts or their reactive-derivative radicals. The compounds adhere to biological substrates such as tooth tissue.
Beech et al., U.S. Pat. No. 4,732,943 discloses an adhesive comprising (a) a condensate of .epsilon.-caprolactone with one or more acrylic monomers containing hydroxy groups and (b) a polymer containing binding groups capable of binding to the dentin.
One major limitation in the prior art has been the difficulty of adhering to hard tissue which is not dry. This necessitates maintaining a dry tooth surface during the priming procedure which is in practice difficult to ensure. For instance, the oral cavity is inherently humid and the hard tissue is susceptible to fluid perfusion from the pulp chamber. The susceptibility of the tooth surface to this fluid perfusion is believed to be a function of the proximity of the prepared surface to the pulp chamber. Near the pulp, the tubules are very close together and the water content of this deep dentin is very high. See, Pashley, Dentin: A Dynamic Substrate Scanning Microscopy, Vol. 3, No. 1, 161-76 (1989).
Additionally, dental materials adhere poorly to sclerotic dentin and cervical enamel. Sclerotic dentin is characterized as hypermineralized dentin (i.e., the dentinal tubular contents are mineralized) and has a coloration that can range from transparent to intense yellow or yellow-brown.